(1) Field of the Invention
The invention relates to the prevention of contact autodoping and tungsten silicide peeling during the reflow cycle for a borophosphosilicate glass, or BPSG, insulating layer. The contact autodoping and tungsten silicide peeling are avoided by means of a thin oxide layer which protects contact areas during the reflow cycle.
(2) Description of Related Art
The problem of autodoping during the BPSG reflow cycle has been addressed by others. A flow of oxygen gas can be introduced during the reflow cycle to form a SiO.sub.2 layer on the contacts can prevent autodoping but will cause peeling of tungsten silicide due to depletion of the silicon in the tungsten silicide layer. The tungsten silicide layer has gone through several thermal cycles due to process requirements prior to the reflow cycle resulting in serious silicon depletion.
As deposited the tungsten silicide layer is WSi.sub.2.6-2.7 and after tungsten silicide annealing, which is necessary to reduce the resistance of the tungsten silicide layer the tungsten silicide layer becomes WSi.sub.2.3-2.4. If the tungsten silicide layer is exposed to an oxidizing environment oxidation depletes silicon from the tungsten silicide layer which becomes WSi.sub.2.2. These changes affect the crystal structure of the tungsten silicide and cause stress increases which result in peeling of the tungsten silicide.
Changes in the WSi.sub.x film are discussed in the paper "Si/W Ratio Changes and Film Peeling During Polycide Anneal," by C. S. Yoo, Japanese Journal of Applied Physics, vol. 29, Nov. 1990, pages 2535-2540. U.S. Pat. No. 5,089,432 to C. S. Yoo provides a method for capping tungsten silicide with a thin oxide layer.
This invention uses a thin oxide layer to protect contact areas during the BPSG reflow cycle. The thickness of the oxide layer is an important part of the invention as it must be thin enough to allow the BPSG reflow to occur as well as not impact subsequent processing, such as pre-metal contact etch time prior to first metal sputtering, and thick enough to protect the contact areas during the reflow cycle.